This invention relates to a method for recovery of a dissolved solid from a solution containing it.
In a particular aspect, this invention relates to a method for recovering crystalline nitroalkanol from a solution containing it.
Nitroalkanols, including the diols and the triols, are readily prepared by condensing a nitroalkane with an aldehyde in the presence of an alkaline catalyst, as is known. The reaction is usually conducted in an aqueous medium and the product is obtained as an aqueous solution of the nitroalkanol. It is recovered by concentrating and cooling the aqueous solution to crystallize the nitroalkanol.
Four nitroalkanes of from 1 to 3 carbon atoms are commercially available and from these, five nitroalkanols are commercially available, obtained by condensing the nitroalkane with one or more moles of formaldehyde. Of these five compounds, four are crystalline solids and one, 2-nitro-1-butanol, is a liquid. The solid compounds do not liquify at elevated temperatures but rather melt with decomposition.
The commercial crystalline nitroalkanols are highly soluble in water. For example, following are their solubilities at 20.degree. C. in 100 g of water. EQU 2-Nitro-2-methyl-1-propanol 350 g EQU 2-Nitro-2-methyl-1,3-propanediol 80 g EQU 2-Nitro-2-ethyl-1,3-propanediol 400 g EQU Tris(hydroxymethyl)nitromethane 200 g
Previously these compounds have been commercially prepared by crystallizing them from aqueous solution. However, due to the high solubility in water, the yields are poor and a significant waste disposal problem exists. R. F. Cox, U.S. Pat. No. 2,347,312, disclosed a process which gave improved yields of crystalline tris(hydroxymethyl)nitromethane, hereinafter designated TN, by condensing nitromethane and formaldehyde in the presence of an organic solvent so that the TN remained in solution at the reaction temperature, but crystallized when chilled. The solvents employed included lower aliphatic alcohols but such mixtures as ethyl acetate with hexane and butyl alcohol with toluene were suggested.
This process suffered from two disadvantages. The principal one was that the reaction was much slower in organic solvent than in aqueous solution and the solubility even when chilled was still significant. Consequently, the problems of low yield and disposal of wastes continued. Furthermore, the longer reaction time lowered the productivity and raised the energy requirements. Accordingly, there is a need for an improved method recovering these highly soluble compounds from solutions containing them.